The suprathermal electron contributions to high‐latitude Birkeland Currents

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Abstract
Observations obtained from instruments on the Isis 2 spacecraft in polar orbit at 1400 km have been used to investigate the identity of the charge carriers in the large‐scale field‐aligned current systems present in the high‐latitude region. The vertical current is inferred from observed perturbations to the east‐west component of the local geomagnetic field obtained by the magnetometer. Simultaneous observations of the charged particle flux were obtained by the soft particle spectrometer (in the 0.005‐ to 15‐kev range) and by the retarding potential analyzer in the suprathermal energy range (≧1 eV). The flux of electrons in the suprathermal energy range has been studied in an attempt to identify and obtain consistency between the velocity distribution function of the charge carriers and the inferred currents. The observed flow of upwelling suprathermal electrons increased substantially when currents were inferred of either direction. The mean net current density due to suprathermal electrons was 0.4±0.2 µA/m² when the expected current was upward, compared to a mean expected value of 0.2±0.1 µA/m² inferred from the magnetometer. This mean net current density was derived by combining the observed much larger downward flux of electrons, primarily energetic, with an observed upward flux of suprathermal electrons. The partial cancellation of the charge transfer due to the precipitating particles by a return current of suprathermals results in reasonable agreement between the observed net particle flow and the magnetic field perturbation. When the expected field‐aligned currents were downward, the mean current density due to suprathermal electrons was −0.8±0.2 µA/m², compared to a magnetometer inferred value of −0.4±0.1 µA/m². The suprathermal electrons therefore contribute substantially to field‐aligned currents, producing most, if not all, of the inward currents.